Tatsumi Hirano

First In Situ Observation of the LiCoO2 Electrode/Electrolyte Interface by Total-Reflection X-ray Absorption Spectroscopy†

Rechargeable lithium-ion batteries (LIBs) are widely used as electrical energy storage devices for technologies such as portable electronics and electric and hybrid vehicles, and they are considered to be serious power storage candidates for smart-grid electricity systems. Traditionally, research in the field has focused on battery improvement through a selective use of new or existing materials for positive and negative electrodes, as the bulk properties of electrodes primarily limit charge capacity and power. However, the durability of LIBs is largely rooted in side reactions that occur at the electrode/ electrolyte interface, especially those at the positive electrode. Thus, controlling the chemical stability of any electrode material with respect to the operating liquid electrolyte medium, which requires a control of the electrode/electrolyte interface through surface chemistry, is as important as designing new materials. The scale of such an interfacial region is speculated to be on the order of a few nanometers, which shall be deemed as approximately the Debye length. This scale indicates that structural and chemical information should be tracked with a resolution of a few nanometers to reveal the phenomena of the electrode/ electrolyte interface. Previous research has focused on a detailed examination of the interfacial reactions at the positive electrode surface by using methods such as X-ray photoelectron spectroscopy (XPS) and surface X-ray diffraction (SXRD). However, characterization of the electrode surface at the nanoscale under conditions of an operating battery remains insufficient because of the lack of suitable observation techniques. A proposed degradation mechanism for electrodes has been extrapolated from indirect information obtained from analysis of disassembled, deteriorated electrodes. To obtain concise and meaningful surface data, a technique that enables high-resolution analysis of chemical information at the solid electrode surface is required. X-ray absorption spectroscopy (XAS), which makes it possible to identify the electronic and local structures of a certain atom, is a potent and versatile technique to resolve the chemical states of a lithium-ion electrode material independently of its crystallinity. To extract information about the interfacial phenomena by XAS, total-reflection fluorescence XAS (TRF-XAS), which integrates the fluorescence yield obtained under total reflection, can be applied. A recent study has shown that polycrystalline thin films are preferred relative to epitaxial thin films (that are strongly influenced by the substrate) to simulate the conditions of applied composite electrodes. We herein use polycrystalline LiCoO2 thin films prepared by pulsed laser deposition (PLD) as the model electrodes; these electrodes are flat at the nanoscale and have structural properties similar to those of the applied composite electrode (see section S1 in the Supporting Information). Figure 1 shows the charge/discharge cycle dependencies of cyclic voltammograms (CVs) and electrochemical impedance spectra (EIS) of the LiCoO2 thin films used in this study (see section S1 in the Supporting Information). Typical CVs

IN SITU X-RAY CT UNDER TENSILE LOADING USING SYNCHROTRON RADIATION

Internal damage in metal matrix composite (MMC) under static tensile loading was observed by in situ x-ray computed tomography based on synchrotron radiation (SR-CT). A tensile testing sample stage was developed to investigate the fracture process during the tensile test. Aluminum alloy matrix composites reinforced by long or short SiC fibers were used. The projection images obtained under tensile loading showed good performance of the sample stage, and matrix deformation and breaks of the long SiC fibers could be observed. In the CT images taken at the maximum stress just before failure, debondings of the short SiC fibers to the matrix, many pullouts of the fibers, and matrix cracking could be clearly observed. The in situ SR-CT allowed the observation of generation and growth of such defects under different tensile stress levels. The results from the nondestructive observation revealed that the MMC was broken by propagation of the matrix cracks which might be caused by stress concentration at the ends of the short fibers. A three-dimensional CT image reconstructed from many CT images provided easy understanding of the fiber arrangement, crack shape, and form of the void caused by fiber pullout. In situ SR-CT is a useful method for understanding failure mechanisms in advanced materials.

Roughness Measurement of X-Ray Mirror Surfaces

In order to fabricate an X-ray mirror telescope, we investigated the roughness of mirror surfaces using X-ray scattering. With Al-K (8.34A) X-rays we measured the scattering profiles of plate glass. The spectral structure of surface roughness was revealed by the angular distribution of the X-ray scattering. The power spectral density functions of the surface-height distribution for these materials (except for a gold evapolated surface) were represented by the power-law spectra with power indices ranging from -1 to -2. The rms heights were derived to be 1.8-8.3A for a wavelength range as expected from the power-law spectrum. The results obtained with this method were found to be consistent with those with an ordinary optical profilometer.

Observation of Compositional Separation in CoCrTa Thin Film Using Transmission Electron Microscope with Imaging Filter

The elemental mapping of a CoCrTa thin film deposited at 270° C has been performed using a transmission electron microscope equipped with an imaging filter. A compositional separation, which consists of Co-enriched areas and a Cr-enriched phase around them, is quantitatively observed. The Co-enriched areas are about 20 nm in diameter and the Cr-enriched phase extends with a two-dimensional network, which corresponds to grain boundaries. Intragrain Cr-enriched areas, which are less enriched than at the grain boundaries, are also observed.

Highly efficient and long life metal–insulator–metal cathodes

The authors improved the emission efficiency and lifetime of metal–insulator–metal cathodes. The drift of the diode current was suppressed by using a thinner tunneling insulator and a lower diode voltage. The cathode with a 7.9-nm-thick tunneling insulator kept the diode current stable at 0.5 A/cm2 for more than 20 000 h, although the initial emission efficiency declined from 2% to less than 0.5%, and the emission current drift increased. The decreased emission efficiency could be enhanced to more than 3% by mixing CsHCO3 into an Au/Pt/Ir multilayer top electrode.

X-ray computerized tomography using monochromated synchrotron radiation

An X-ray computerized tomographic (CT) imaging system using monochromated synchrotron radiation has been constructed and its performance has been evaluated. Spatial resolution is about 125 µm. Several CT images have been taken. Element mapping in a test sample has also been obtained using the difference between two CT images at different X-ray energies just above and below the absorption edge of the element.

Improvement of Spatial Resolution of Monochromatic X-ray CT Using Synchrotron Radiation

X-ray computed tomography using synchrotron radiation (SR) as X-ray source potentially provides CT images of improved quality, since SR produces tunable, monochromatized and collimated X-ray beams of high flux density. We can obtain CT images of small samples rapidly with better spatial resolution and contrast; we can also obtain the spatial distribution of a specific element using the absorption edge. In this paper, a method for improving the spatial resolution of such a system is presented, involving the magnification of projection images using asymmetric diffraction. First, the modulation transfer function (MTF) of projection images of an X-ray test chart is measured in order to evaluate the spatial resolution. Next, using this method, we show CT images of several industrial materials with spatial resolution of 15~30 µm and demonstrate the usefulness of SR-CT with a high spatial resolution.

X‐ray sensing pickup tube

An x‐ray sensing pickup tube free of white defects in the scanning area has been developed, and its characteristics have been evaluated. This pickup tube functions in a wide x‐ray energy range from 7 to 40 keV. The maximum spectral sensitivity observed at 20 keV photon energy is 6×10−17 A/(photon/s). The limiting resolution is about 8 μm. The observation of a focused x‐ray beam has been carried out.

High resolution monochromatic tomography with x‐ray sensing pickup tube

Using monochromatic synchrotron radiation and a high resolution x‐ray sensing pickup tube as a two‐dimensional detector, we have improved the spatial resolution of x‐ray computed tomographic (CT) images. A quantitative elemental analysis was made based on the difference between two CT images measured just above and below an element’s absorption edge energy. The developed CT imaging system has been used to observe nondestructive fine structures, such as cracks, in a ceramic material. High resolution of 10 μm was obtained. The differential CT value, which was seen as the shading intensity in the differential CT image, was proportional to the concentration of the specific element.

Study of quantitative elemental analysis of monochromatic X-ray CT using synchrotron radiation

X-ray computed tomography (CT), based on monochromatized synchrotron radiation as an X-ray source, provides spatial distributions of a specific element using the difference between two CT images taken just above and below the absorption edge energy of the element. We have studied a quantitative analytical function for an element by measuring X-ray CT images of a model sample using the subtraction method. The differential CT value, which is seen as the shading intensity in the differential CT image, is proportional to the concentration of the specific element. Both spatial distributions and concentrations of the specific element can be nondestructively observed. Barium concentrations in an optical waveguide were obtained, and the relation between the concentration and the refractive index was examined.